Conventionally, flash memories, ferroelectric random access memories (FeRAMs), and magnetoresistive random access memories (MRAMs) have been proposed and used as nonvolatile memories.
However, these conventional nonvolatile memories have encountered various problems as the device size becomes smaller and the integration density becomes higher.
For example, flash memories of the 32-nm node generation and beyond have encountered physical and intrinsic problems such as an increase in the crosstalk between cells, a decrease in the capacitive coupling between a floating gate and a control gate, and the reliability of a tunnel oxide film.
Further, in ferroelectric random access memories, the area of a capacitor that retains information has been reduced with reduction in size, so that it has become more difficult to write and read information stably and to retain information.
Further, magnetoresistive random access memories, which are current-driven devices, cannot avoid a sharp increase in power consumption when the integration density increases with reduction in size.
On the other hand, so-called resistive random access memories (ReRAMs), which use the voltage-induced resistance switching phenomenon found in binary transition metal oxides (TMOs) such as TiO2 and NiO, have been proposed lately. (See, for example, below-listed Non-Patent Documents 1 through 3.)
FIG. 1A is a cross-sectional view of one of such resistive random access memories, providing an overview of its configuration. FIG. 1B is a diagram illustrating an operation of the resistive random access memory of FIG. 1A.
Referring to FIG. 1A, the resistive random access memory is a so-called unipolar type, and includes a lower electrode 11; a resistive film 12 of TiO2, NiO, or the like, formed on the lower electrode 11; and an upper electrode 13 formed on the resistive film 12.
The transition metal oxide forming such a resistive film as the resistive film 12, for example, TiO2 or NiO, normally does not present electrical conductivity and forms an insulator. It is known, however, that when such a device as illustrated in FIG. 1A is formed, and a high voltage is applied between the electrodes 11 and 13 to cause soft breakdown (called “forming”), a bi-stable condition thereafter appears where the resistance of the resistive film 12 switches between a high resistance state (HRS) and a low resistance state (LRS) as illustrated in FIG. 1B. (See, for example, below-listed Non-Patent Documents 1 through 3.)
Referring to FIG. 1B, such resistance switching of the resistive film 12 is induced symmetrically when the polarity of the voltage applied between the electrodes 11 and 13 is reversed. Accordingly, such a device is referred to as a unipolar device.
The mechanism of this phenomenon has not been clarified completely. It is considered, however, that this is because defects such as oxygen deficiencies are aligned to form an electrically conductive filament 12f in the resistive film 12 of a transition metal oxide as a result of the forming as illustrated in FIG. 1A.
That is, the resistive film 12 is believed to be in the low resistance state when this filament 12f is continuous and to be in the high resistance state when this filament 12f is discontinuous.
It is known that when the electrically conductive filament 12f is formed in the resistive film 12 by the forming process and the resistive film 12 is in the high resistance state in the device of FIG. 1A, the resistive film 12 is “set,” that is, switches to the low resistance state, if the voltage applied between the electrodes 11 and 13 is increased to exceed a predetermined set voltage VSET. This low resistance state is understood to be the result of the electrically conductive filament 12f illustrated in FIG. 1A becoming continuous, and is maintained even after returning the voltage applied between the electrodes 11 and 13 to zero. That is, in the device of FIG. 1A, it is possible to write information “1” or “0” by applying a voltage higher than or equal to the set voltage VSET between the electrodes 11 and 13.
On the other hand, it is known that in the device where the resistive film 12 has switched to the lower resistance state, if the voltage applied between the electrodes 11 and 13 is increased to exceed a predetermined reset voltage VRESET, the resistance of the resistive film 12 sharply increases so that the resistive film 12 is “reset,” that is, switches to the high resistance state. This high resistance state is understood to be the result of the electrically conductive filament 12f illustrated in FIG. 1A becoming discontinuous, and is maintained even after returning the voltage applied between the electrodes 11 and 13 to zero. That is, it is possible to write information “0” or “1” in the device of FIG. 1A. The information thus written is maintained even after supply voltage is shut off. Therefore, the device of FIG. 1A operates as a nonvolatile memory.
Further, in the device of FIG. 1A, it is possible to determine whether the resistive film 12 is in the high resistance state or the low resistance state by applying a voltage lower than the reset voltage VRESET between the electrodes 11 and 13 and detecting a current flowing through the resistive film 12. In other words, it is possible to read information in the nonvolatile memory including the device of FIG. 1A.
It is conventionally known that in such a resistive random access memory, the state transition of the resistive film 12 from the high resistance state to the low resistance state (“set”) occurs in an extremely short period of time of approximately 10 ns, while a change from the low resistance state to the high resistance state (“reset”) takes a very long period of time of approximately 5 μs. (See, for example, Non-Patent Document 2 listed below.)    [Non-Patent Document 1] S. Seo, M. J. Lee, D. H. Seo, E. J. Jeoung, D.-S. Suh, Y. S. Joung, I. K. Yoo, I. R. Hwang, S. H. Kim, I. S. Byun, J.-S. Kim, J. S. Choi, and B. H. Park, “Reproducible resistance switching in polycrystalline NiO films,” Appl. Phys. Lett. 85, 5655 (2004).    [Non-Patent Document 2] I. G. Baek, M. S. Lee, S. Seo, M. J. Lee, D. H. Seo, D.-S. Suh, J. C. Park, S. O. Park, H. S. Kim, I. K. Yoo, U.-In Chung, and J. T. Moon, “Highly Scalable Non-volatile Resistive Memory using Simple Binary Oxide Driven by Asymmetric Unipolar Voltage Pulses,” IEDM Tech. Dig., 2004, p. 587.    [Non-Patent Document 3] B. J. Choi, D. S. Jeong, S. K. Kim, C. Rohde, S. Choi, J. H. Oh, H. J. Kim, C. S. Hwang, K. Szot, R. Waser, B. Reichenberg, and S. Tiedke, “Resistive switching mechanisms of TiO2 thin films grown by atomic-layer deposition,” J. Appl. Phys. 98, 033715 (2005).    [Non-Patent Document 4] A. Beck, J. G. Bednorz, Ch. Gerber, C. Rossel, and D. Widmer, “Reproducible switching effect in thin oxide films for memory applications,” Appl. Phys. Lett. 77, 139 (2000).    [Non-Patent Document 5] Dongsoo Lee, Dong-jun Seong, Inhwa Jo, F. Xiang, R. Dong, Seokjoon Oh, and Hyunsang Hwang, “Resistance switching of copper doped MoOx films for nonvolatile memory applications,” Appl. Phys. Lett. 90, 122104 (2007).